Process of activating anti-microbial materials

ABSTRACT

Anti-microbial coatings and powders and method of forming same on medical devices are provided. The coatings are formed by depositing an anti-microbial biocompatible metal by vapour deposition techniques to produce atomic disorder in the coating such that a sustained release of metal ions sufficient to produce an anti-microbial effect is achieved. Preferred deposition conditions to achieve atomic disorder include a lower than normal substrate temperature, and one or more of a higher than normal working gas pressure and a lower than normal angle of incidence of coating flux. Anti-microbial powders formed by mechanical working to produce atomic disorder are also provided. The anti-microbial effect of the coatings or powders may be further activated or enhanced by irradiating with a low linear energy transfer form of radiation such as gamma radiation.

FIELD OF THE INVENTION

This invention relates to methods for preparing modified materials suchas metal coatings or powders in a form such that metal species arereleased on a sustainable basis at an enhanced rate. In a particularaspect, the invention relates to methods of forming anti-microbialcoatings and powders of biocompatible metals which provide a sustainedrelease of anti-microbial metal species when in contact with body fluidsor body tissues.

BACKGROUND OF THE INVENTION

The need for an effective anti-microbial coating is well established inthe medical community. Physicians and surgeons using medical devices andappliances ranging from orthopaedic pins, plates and implants through towound dressings and urinary catheters must constantly guard againstinfection. An inexpensive anti-microbial coating also finds applicationin medical devices used in consumer healthcare and personal hygieneproducts as well as in biomedical/biotechnical laboratory equipment. Theterm "medical device", as used herein and in the claims is meant toextend to all such products.

The anti-microbial effects of metallic ions such as Ag, Au, Pt, Pd, Ir(i.e. the noble metals), Cu, Sn, Sb, Bi and Zn are known (see Morton,H.E., Pseudomonas in Disinfection, Sterilization and Preservation, ed.S. S. Block, Lea and Febiger, 1977 and Grier, N., Silver and ItsCompounds in Disinfection, Sterilization and Preservation, ed. S. S.Block, Lea and Febiger, 1977). Of the metallic ions with anti-microbialproperties, silver is perhaps the best known due to its unusually goodbioactivity at low concentrations. This phenomena is termed oligodynamicaction. In modern medical practice both inorganic and organic solublesalts of silver are used to prevent and treat microbial infections.While these compounds are effective as soluble salts, they do notprovide prolonged protection due to loss through removal or complexationof the free silver ions. They must be reapplied at frequent intervals toovercome this problem. Reapplication is not always practical, especiallywhere an in-dwelling or implanted medical device is involved.

Attempts have been make to slow the release of silver ions duringtreatment by creating silver containing complexes which have a lowerlevel of solubility. For example, U.S. Pat. No. 2,785,153 disclosescolloidal silver protein for this purpose. Such compounds are usuallyformulated as creams. These compounds have not found wide applicabilityin the medical area due to their limited efficacy. The silver ionrelease rate is very slow. Furthermore, coatings from such compoundshave been limited due to adhesion, abrasion resistance and shelf lifeproblems.

The use of silver metal coatings for anti-microbial purposes has beensuggested. For instance, see Deitch et al., Anti-microbial Agents andChemotherapy, Vol. 23(3), 1983, pp. 356-359 and Mackeen et al.,Anti-microbial Agents and Chemotherapy, Vol. 31(1), 1987, pp. 93-99.However, it is generally accepted that such coatings alone do notprovide the required level of efficacy, since diffusion of silver ionsfrom the metallic surface is negligible.

A silver metal coating is produced by Spire Corporation, U.S.A. underthe trade mark SPI-ARGENT. The coating is formed by an ion-beam assisteddeposition (IBAD) coating process. The infection resistant coating isstated to be non-leaching in aqueous solutions as demonstrated by zoneof inhibition tests, thus enforcing the belief that silver metalsurfaces do not release anti-microbial amounts of silver ions.

Given the failure of metallic silver coatings to generate the requiredanti-microbial efficacy, other researchers have tried novel activationprocesses. One technique is to use electrical activation of metallicsilver implants (see Marine et al., Journal of Biological Physics, Vol.12, 1984, pp. 93-98). Electrical stimulation of metallic silver is notalways practical, especially for mobile patients. Attempts to overcomethis problem include developing in situ electrical currents throughgalvanic action. Metal bands or layers of different metals are depositedon a device as thin film coatings. A galvanic cell is created when twometals in contact with each other are placed in an electricallyconducting fluid. One metal layer acts as an anode, which dissolves intothe electrolyte. The second metal acts as a cathode to drive theelectrochemical cell. For example, in the case of alternating layers ofCu and Ag, the Cu is the anode, releasing Cu⁺ ions into the electrolyte.The more noble of the metals, Ag, acts as the cathode, which does notionize and does not go into solution to any large extent. An exemplarydevice of this nature is described in U.S. Pat. No. 4,886,505 issuedDec. 12, 1989, to Haynes et al. The patent discloses sputtered coatingsof two or more different metals with a switch affixed to one of themetals such that, when the switch is closed, metal ion release isachieved.

Previous work has shown that a film composed of thin laminates ofalternating, different metals such as silver and copper can be made todissolve if the surface is first etched. In this instance, the etchingprocess creates a highly textured surface (see M. Tanemura and F.Okuyama, J. Vac. Sci. Technol., 5, 1986, pp 2369-2372). However, theprocess of making such multilaminated films is time consuming andexpensive.

Electrical activation of metallic coatings has not presented a suitablesolution to the problem. It should be noted that galvanic action willoccur only when an electrolyte is present and if an electricalconnection between the two metals of the galvanic couple exists. Sincegalvanic corrosion occurs primarily at the metallic interface betweenthe two metals, electrical contact is not sustained. Thus a continuousrelease of metal ions over an extended period of time is not probable.Also, galvanic action to release a metal such as silver is difficult toachieve. As indicated above, the metal ions exhibiting the greatestanti-microbial effect are the noble metals, such as Ag, Au, Pt and Pd.There are few metals more noble than these to serve as cathode materialsso as to drive the release of a noble metal such as Ag at the anode.

A second approach to activating the silver metal surface is to use heator chemicals. U.S. Pat. Nos. 4,476,590 and 4,615,705, issued to Scaleset al. on Oct. 16, 1984 and Oct. 7, 1986, respectively, disclose methodsof activating silver surface coatings on endoprosthetic implants torender them bioerodible by heating at greater than 180° C. or bycontacting with hydrogen peroxide. Such treatments are limited in termsof the substrate/devices which can be coated and activated.

There is still a need for an efficacious, inexpensive anti-microbialmaterial having the following properties:

sustained release of an anti-microbial agent at therapeutically activelevels;

applicable to a wide variety of devices and materials;

useful shelf life; and

low mammalian toxicity.

Metal coatings are typically produced as thin films by vapour depositiontechniques such as sputtering. Thin films of metals, alloys,semiconductors and ceramics are widely used in the production ofelectronic components. These and other end uses require the thin filmsto be produced as dense, crystalline structures with minimal defects.The films are often annealed after deposition to enhance grain growthand recrystallization and produce stable properties. Techniques todeposit metal films are reviewed by R. F. Bunshah et al., "DepositionTechnologies for Films and Coatings", Noyes Publications, N.J., 1982 andby J. A. Thornton, "Influence of Apparatus Geometry and DepositionConditions on the Structure and Topography of Thick Sputtered Coatings",J. Vac. Sci. Technol., 11(4), 666-670, 1974.

U.S. Pat. No. 4,325,776, issued Apr. 20, 1982 to Menzel discloses aprocess for producing coarse or single crystal metal films from certainmetals for use in integrated circuits. The metal film is formed bydepositing on a cooled substrate (below-90° C.) such that the metallayer is in an amorphous phase. The metal layer is then annealed byheating the substrate up to about room temperature. The end product isstated to have large grain diameter and great homogeneity, permittinghigher current densities without electromigration failures.

SUMMARY OF THE INVENTION

The inventors set out to develop an anti-microbial metal coating. Theydiscovered that, contrary to previous belief, it is possible to formmetal coatings from an anti-microbial metal material by creating atomicdisorder in the materials by vapour deposition under conditions whichlimit diffusion, that is which "freeze-in" the atomic disorder. Theanti-microbial coatings so produced were found to provide sustainedrelease of anti-microbial metal species into solution so as to producean anti-microbial effect.

This basic discovery linking "atomic disorder" to enhanced solubilityhas broad application. The inventors have demonstrated that atomicdisorder so as to produce solubility can be created in other materialforms, such as metal powders. The invention also has application beyondanti-microbial metals, encompassing any metal, metal alloy, or metalcompound, including semiconductor or ceramic materials, from whichsustained release of metal species into solution is desired. Forinstance, materials having enhanced or controlled metal dissolution findapplication in sensors, switches, fuses, electrodes, and batteries.

The term "atomic disorder" as used herein includes high concentrationsof: point defects in a crystal lattice, vacancies, line defects such asdislocations, interstitial atoms, amorphous regions, grain and sub grainboundaries and the like relative to its normal ordered crystallinestate. Atomic disorder leads to irregularities in surface topography andinhomogenieties in the structure on a nanometer scale.

By the term "normal ordered crystalline state" as used herein is meantthe crystallinity normally found in bulk metal materials, alloys orcompounds formed as cast, wrought or plated metal products. Suchmaterials contain only low concentrations of such atomic defects asvacancies, grain boundaries and dislocations.

The term "diffusion" as used herein implies diffusion of atoms and/ormolecules on the surface or in the matrix of the material being formed.

The terms "metal" or "metals" as used herein are meant to include one ormore metals whether in the form of substantially pure metals, alloys orcompounds such as oxides, nitrides, borides, sulphides, halides orhydrides.

The invention, in a broad aspect extends to a method of forming amodified material containing one or more metals. The method comprisescreating atomic disorder in the material under conditions which limitdiffusion such that sufficient atomic disorder is retained in thematerial to provide release, preferably on a sustainable basis, ofatoms, ions, molecules or clusters of at least one of the metals into asolvent for the material. Clusters are known to be small groups ofatoms, ions or the like, as described by R. P. Andres et al., "ResearchOpportunities on Clusters and Cluster-Assembled Materials", J. Mater.Res. Vol. 4, No. 3, 1989, P. 704.

Specific preferred embodiments of the invention demonstrate that atomicdisorder may be created in metal powders or foils by cold working, andin metal coatings by depositing by vapour deposition at low substratetemperatures.

In another broad aspect, the invention provides a modified materialcomprising one or more metals in a form characterized by sufficientatomic disorder such that the material, in contact with a solvent forthe material, releases atoms, ions, molecules or clusters containing atleast one metal, preferably on a sustainable basis, at an enhanced raterelative to its normal ordered crystalline state.

In preferred embodiments of the invention, the modified material is ametal powder which has been mechanically worked or compressed, undercold working conditions, to create and retain atomic disorder.

The term "metal powder" as used herein is meant to include metalparticles of a broad particle size, ranging from nanocrystalline powdersto flakes.

The term "cold working" as used herein indicates that the material hasbeen mechanically worked such as by milling, grinding, hammering, mortarand pestle or compressing, at temperatures lower than therecrystallization temperature of the material. This ensures that atomicdisorder imparted through working is retained in the material.

In another preferred embodiment, the modified material is a metalcoating formed on a substrate by vapour deposition techniques such asvacuum evaporation, sputtering, magnetron sputtering or ion plating. Thematerial is formed under conditions which limit diffusion duringdeposition and which limit annealing or recrystallization followingdeposition. The deposition conditions preferably used to produce atomicdisorder in the coatings are outside the normal range of operatingconditions used to produce defect free, dense, smooth films. Such normalpractices are well known (see for example R. F. Bunshah et al., supra).Preferably the deposition is conducted at low substrate temperaturessuch that the ratio of the substrate to the melting point of the metalor metal compound being deposited (T/Tm) is maintained at less thanabout 0.5, more preferably at less than about 0.35, and most preferablyat less than 0.30. In this ratio, the temperatures are in degreesKelvin. The preferred ratio will vary from metal to metal and increaseswith alloy or impuritiy content. Other preferred deposition conditionsto create atomic disorder include one or more of a higher than normalworking gas pressure, a lower than normal angle of incidence of thecoating flux and a higher than normal coating flux.

The temperature of deposition or cold working is not so low thatsubstantial annealing or recrystallization will take place when thematerial is brought to room temperature or its intended temperature foruse (ex. body temperature for anti-microbial materials). If thetemperature differential between deposition and temperature of use (ΔT)is too great, annealing results, removing atomic disorder. This ΔT willvary from metal to metal and with the deposition technique used. Forexample, with respect to silver, substrate temperatures of -20° to 200°C. are preferred during physical vapour deposition.

Normal or ambient working gas pressure for depositing the usuallyrequired dense, smooth, defect free metal films vary according to themethod of physical vapour deposition being used. In general, forsputtering, the normal working gas pressure is less than 75 mT(milliTorr), for magnetron sputtering, less than 10 mT, and forion-plating less than 200 mT. Normal ambient gas pressures vary forvacuum evaporation processes vary as follows: for e-beam or arcevaporation, from 0.001 to 0.01 mT; for gas scattering evaporation(pressure plating) and reactive arc evaporation, up to 200 mT, buttypically less than 20 mT. Thus, in accordance with the method of thepresent invention, in addition to using low substrate temperatures toachieve atomic disorder, working (or ambient) gas pressures higher thanthese normal values may be used to increase the level of atomic disorderin the coating.

Another condition discovered to have an effect on the level of atomicdisorder in the coatings of the present invention is the angle ofincidence of the coating flux during deposition. Normally to achievedense, smooth coatings, this angle is maintained at about 90°+/-15°. Inaccordance with the present invention, in addition to using lowsubstrate temperatures during deposition to achieve atomic disorder,angles of incidence lower than about 75° may be used to increase thelevel of atomic disorder in the coating.

Yet another process parameter having an effect on the level of atomicdisorder is the atom flux to the surface being coated. High depositionrates tend to increase atomic disorder, however, high deposition ratesalso tend to increase the coating temperature. Thus, there is an optimumdeposition rate that depends on the deposition technique, the coatingmaterial and other process parameters.

To provide an anti-microbial material, the metals used in the coating orpowder are those which have an anti-microbial effect, but which arebiocompatible (non-toxic for the intended utility). Preferred metalsinclude Ag, Au, Pt, Pd, Ir (i.e. the noble metals), Sn, Cu, Sb, Bi, andZn, compounds of these metals or alloys containing one more of thesemetals. Such metals are hereinafter referred to as "anti-microbialmetals"). Most preferred is Ag or its alloys and compounds.Anti-microbial materials in accordance with this invention preferablyare formed with sufficient atomic disorder that atoms, ions, moleculesor clusters of the anti-microbial material are released into an alcoholor water based electrolyte on a sustainable basis. The terms"sustainable basis" is used herein to differentiate, on the one handfrom the release obtained from bulk metals, which release metal ions andthe like at a rate and concentration which is too low to achieve ananti-microbial effect, and on the other hand from the release obtainedfrom highly soluble salts such as silver nitrate, which release silverions virtually instantly in contact with an alcohol or water basedelectrolyte. In contrast, the anti-microbial materials of the presentinvention release atoms, ions, molecules or clusters of theanti-microbial metal at a sufficient rate and concentration, over asufficient time period to provide a useful anti-microbial effect.

The term "anti-microbial effect" as used herein means that atoms, ions,molecules or clusters of the anti-microbial metal are released into theelectrolyte which the material contacts in concentrations sufficient toinhibit bacterial growth in the vicinity of the material. The mostcommon method of measuring anti-microbial effect is by measuring thezone of inhibition (ZOI) created when the material is placed on abacterial lawn. A relatively small or no ZOI (ex. less than 1 mm)indicates a non useful anti-microbial effect, while a larger ZOI (ex.greater than 5 mm) indicates a highly useful anti-microbial effect. Oneprocedure for a ZOI test is set out in the Examples which follow.

The invention extends to devices such as medical devices formed from,incorporating, carrying or coated with the anti-microbial powders orcoatings. The anti-microbial coating may be directly deposited by vapourdeposition onto such medical devices as catheters, sutures, implants,burn dressings and the like. An adhesion layer, such as tantalum, may beapplied between the device and the anti-microbial coating. Adhesion mayalso be enhanced by methods known in the art, for example etching thesubstrate or forming a mixed interface between the substrate and thecoating by simultaneous sputtering and etching. Anti-microbial powdersmay be incorporated into creams, polymers, ceramics, paints, or othermatrices, by techniques well known in the art.

In a further broad aspect of the invention, modified materials areprepared as composite metal coatings containing atomic disorder. In thiscase, the coating of the one or more metals or compounds to be releasedinto solution constitutes a matrix containing atoms or molecules of adifferent material. The presence of different atoms or molecules resultsin atomic disorder in the metal matrix, for instance due to differentsized atoms. The different atoms or molecules may be one or more secondmetals, metal alloys or metal compounds which are co or sequentiallydeposited with the first metal or metals to be released. Alternativelythe different atoms or molecules may be absorbed or trapped from theworking gas atmosphere during reactive vapour deposition. The degree ofatomic disorder, and thus solubility, achieved by the inclusion of thedifferent atoms or molecules varies, depending on the materials. Inorder to retain and enhance the atomic disorder in the compositematerial, one or more of the above-described vapour depositionconditions, namely low substrate temperature, high working gas pressure,low angle of incidence and high coating flux, may be used in combinationwith the inclusion of different atoms or molecules.

Preferred composite materials for anti-microbial purposes are formed byincluding atoms or molecules containing oxygen, nitrogen, hydrogen,boron, sulphur or halogens in the working gas atmosphere whiledepositing the anti-microbial metal. These atoms or molecules areincorporated in the coating either by being absorbed or trapped in thefilm, or by reacting with the metal being deposited. Both of thesemechanisms during deposition are hereinafter referred to as "reactivedeposition". Gases containing these elements, for example oxygen,hydrogen, and water vapour, may be provided continuously or may bepulsed for sequential deposition.

Anti-microbial composite materials are also preferably prepared by co-or sequentially depositing an anti-microbial metal with one or moreinert biocompatible metals selected from Ta, Ti, Nb, Zn, V, Hf, Mo, Si,and Al. Alternatively, the composite materials may be formed by co-,sequentially or reactively depositing one or more of the anti-microbialmetals as the oxides, carbides, nitrides, borides, sulphides or halidesof these metals and/or the oxides, carbides, nitrides, borides,sulphides or halides of the inert metals. Particularly preferredcomposites contain oxides of silver and/or gold, alone or together withone or more oxides of Ta, Ti, Zn and Nb.

The invention also extends to a method of activating or furtherenhancing the anti-microbial effect of anti-microbial materials formedwith atomic disorder. Thus, anti-microbial materials made in accordancewith the present invention may be irradiated to further enhance theanti-microbial effect. However, it is also possible to irradiatematerials initially formed with a level of atomic disorder which isinsufficient to produce an anti-microbial effect, such that theirradiated material has an acceptable anti-microbial effect. The processof activation comprises irradiating the material with a low linearenergy transfer form of radiation such as beta or x-rays, but mostpreferably gamma rays. A dose greater than 1 Mrad is preferred. Theanti-microbial material is preferably oriented substantiallyperpendicular to the incoming radiation. The level of activation may befurther enhanced by irradiating the material adjacent to a dielectricmaterial such as oxides of Ta, Al and Ti, but most preferably siliconoxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As above stated, the present invention has application beyondanti-microbial materials. However, the invention is disclosed hereinwith anti-microbial metals, which are illustrative of utility for othermetals, metal alloys and metal compounds. Preferred metals include Aland Si, and the metal elements from the following groups of the periodictable: IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA, and VA(excluding As) in the periods 4, 5 and 6, (see Periodic Table aspublished in Merck Index 10th Ed., 1983, Merck and Co. Inc., Rahway,N.J., Martha Windholz). Different metals will have varying degrees ofsolubility. However, the creation and retention of atomic disorder inaccordance with this invention results in enhanced solubility (release)of the metal as ions, atoms, molecules or clusters into an appropriatesolvent i.e. a solvent for the particular material, typically a polarsolvent, over the solubility of the material in its normal orderedcrystalline state.

The medical devices formed from, incorporating, carrying or coated withthe anti-microbial material of this invention generally come intocontact with an alcohol or water based electrolyte including a bodyfluid (for example blood, urine or saliva) or body tissue (for exampleskin, muscle or bone) for any period of time such that microorganismgrowth on the device surface is possible. The term "alcohol or waterbased electrolyte" also includes alcohol or water based gels. In mostcases the devices are medical devices such as catheters, implants,tracheal tubes, orthopacdic pins, insulin pumps, wound closures, drains,dressings, shunts, connectors, prosthetic devices, pacemaker leads,needles, surgical instruments, dental prostheses, ventilator tubes andthe like. However, it should be understood that the invention is notlimited to such devices and may extend to other devices useful inconsumer healthcare, such as sterile packaging, clothing and footwear,personal hygiene products such as diapers and sanitary pads, inbiomedical or biotechnical laboratory equipment, such as tables,enclosures and wall coverings, and the like. The term "medical device"as used herein and in the claims is intended to extend broadly to allsuch devices.

The device may be made of any suitable material, for example metals,including steel, aluminum and its alloys, latex, nylon, silicone,polyester, glass, ceramic, paper, cloth and other plastics and rubbers.For use as an in-dwelling medical device, the device will be made of abioinert material. The device may take on any shape dictated by itsutility, ranging from flat sheets to discs, rods and hollow tubes. Thedevice may be rigid or flexible, a factor again dictated by its intendeduse.

Anti-Microbial Coatings

The anti-microbial coating in accordance with this invention isdeposited as a thin metallic film on one or more surfaces of a medicaldevice by vapour deposition techniques. Physical vapour techniques,which are well known in the art, all deposit the metal from the vapour,generally atom by atom, onto a substrate surface. The techniques includevacuum or are evaporation, sputtering, magnetron sputtering and ionplating. The deposition is conducted in a manner to create atomicdisorder in the coating as defined hereinabove. Various conditionsresponsible for producing atomic disorder are useful. These conditionsare generally avoided in thin film deposition techniques where theobject is to create a defect free, smooth and dense film (see forexample J. A. Thornton, supra). While such conditions have beeninvestigated in the art, they have not heretofore been linked toenhanced solubility of the coatings so-produced.

The preferred conditions which are used to create atomic disorder duringthe deposition process include:

a low substrate temperature, that is maintaining the surface to becoated at a temperature such that the ratio of the substrate temperatureto the melting point of the metal (in degrees Kelvin) is less than about0.5, more preferably less than about 0.35 and most preferably less thanabout 0.3; and optionally one or both of:

a higher than normal working (or ambient) gas pressure, i.e., for vacuumevaporation: e-beam or arc evaporation, greater than 0.01 mT, gasscattering evaporation (pressure plating) or reactive are evaporation,greater than 20 mT; for sputtering: greater than 75 ml; for magnetronsputtering: greater than about 10 mT; and for ion plating: greater thanabout 200 mT; and

maintaining the angle of incidence of the coating flux on the surface tobe coated at less than about 75°, and preferably less than about 30°

The metals used in the coating are those known to have an ant-microbialeffect. For most medical devices, the metal must also be biocompatible.Preferred metals include the noble metals Ag, Au, Pt, Pd, and Ir as wellas Sn, Cu, Bi, and Zn or alloys or compounds of these metals or othermetals. Most preferred is Ag or Au, or alloys or compounds of one ormore of these metals.

The coating is formed as a thin film on at least a part of the surfaceof the medical device. The film has a thickness no greater than thatneeded to provide release of metal ions on a sustainable basis over asuitable period of time. In that respect, the thickness will vary withthe particular metal in the coating (which varies the solubility andabrasion resistance), and with the degree of atomic disorder in (andthus the solubility of) the coating. The thickness will be thin enoughthat the coating does not interfere with the dimensional tolerances orflexibility of the device for its intended utility. Typically, thicknesof less than 1 micron have been found to provide sufficient sustainedanti-microbial activity. Increased thicknesses may be used depending onthe degree of metal ion release needed over a period of time. Thicknessgreater than 10 microns are more expensive to produce and normallyshould not be needed.

The anti-microbial effect of the coating is achieved when the device isbrought into contact with an alcohol or a water based electrolyte suchas, a body fluid or body tissue, thus releasing metal ions, atoms,molecules or clusters. The concentration of the metal which is needed toproduce an anti-microbial effect will vary from metal to metal.Generally, anti-microbial effect is achieved in body fluids such asplasma, serum or urine at concentrations less than about 0.5-1.5 μg/ml.

The ability to achieve release of metal atoms, ions, molecules orclusters on a sustainable basis from a coating is dictated by a numberof factors, including coating characteristics such as composition,structure, solubility and thickness, and the nature of the environmentin which the device is used. As the level of atomic disorder isincreased, the amount of metal ions released per unit time increases.For instance, a silver metal film deposited by magnetron sputtering atT/Tm<0.5 and a working gas pressure of about 7 mTorr releasesapproximately 1/3 of the silver ions that a film deposited under similarconditions, but at 30 mTorr, will release over 10 days. Films that arecreated with an intermediate structure (ex. lower pressure, lower angleof incidence etc.) have Ag release values intermediate to these valuesas determined by bioassays. This then provides a method for producingcontrolled release metallic coatings in accordance with this invention.Slow release coatings are prepared such that the degree of disorder islow while fast release coatings are prepared such that the degree ofdisorder is high.

For continuous, uniform coatings, the time required for totaldissolution will be a function of film thickness and the nature of theenvironment to which they are exposed. The relationship in respect ofthickness is approximately linear, i.e. a two fold increase in filmthickness will result in about a two fold increase in longevity.

It is also possible to control the metal release from a coating byforming a thin film coating with a modulated structure. For instance, acoating deposited by magnetron sputtering such that the working gaspressure was low (ex. 15 mTorr) for 50% of the deposition time and high(ex. 30 mTorr) for the remaining time, has a rapid initial release ofmetal ions, followed by a longer period of slow release. This type ofcoating is extremely effective on devices such as urinary catheters forwhich an initial rapid release is required to achieve immediateanti-microbial concentrations followed by a lower release rate tosustain the concentration of metal ions over a period of weeks.

The substrate temperature used during vapour deposition should not be solow that annealing or recrystallization of the coating takes place asthe coating warms to ambient temperatures or the temperatures at whichit is to be used (ex. body temperature). This allowable ΔT, that thetemperature differential between the substrate temperature duringdeposition and the ultimate temperature of use, will vary from metal tometal. For the most preferred metals of Ag and Au, preferred substratetemperatures of -20° to 200° C., more preferably -10° C. to 100° C. areused.

Atomic order may also be achieved, in accordance with the presentinvention, by preparing composite metal materials, that is materialswhich contain one or more anti-microbial metals in a metal matrix whichincludes atoms or molecules different from the anti-microbial metals.

Our technique for preparing composite material is to co- or sequentiallydeposit the anti-microbial metal(s) with one or more other inert,biocompatible metals selected from Ta, Ti, Nb, Zn, V, Hf, Mo, Si, Al andalloys of these metals or other metal elements, typically othertransition metals. Such inert metals have a different atomic radii fromthat of the anti-microbial metals, which results in atomic disorderduring deposition. Alloys of this kind can also serve to reduce atomicdiffusion and thus stabilize the disordered structure. Thin filmdeposition equipment with multiple targets for the placement of each ofthe anti-microbial and inert metals is preferably utilized. When layersare sequentially deposited the layer(s) of the inert metal(s) should bediscontinuous, for example as islands within the anti-microbial metalmatrix. The final ratio of the anti microbial metal(s) to inert metal(s)should be greater than about 0.2. The most preferable inert metals areTi, Ta, Zn and Nb. It is also possible to form the anti-microbialcoating from oxides, carbides, nitrides, sulphides, borides, halides orhydrides of one or more of the anti-microbial metals and/or one or moreof the inert metals to achieve the desired atomic disorder.

Another composite material within the scope of the present invention isformed by reactively co- or sequentially depositing, by physical vapourtechniques, a reacted material into the thin film of the anti-microbialmetal(s). The reacted material is an oxide, nitride, carbide, boride,sulphide, hydride or halide of the anti-microbial and/or inert metal,formed in situ by injecting the appropriate reactants, or gasescontaining same, (ex. air, oxygen, water, nitrogen, hydrogen, boron,sulphur, halogens) into the deposition chamber. Atoms or molecules ofthese gases may also become absorbed or trapped in the metal film tocreate atomic disorder. The reactant may be continuously supplied duringdeposition for codeposition or it may be pulsed to provide forsequential deposition. The final ratio of anti-microbial metal(s) toreaction product should be greater than about 0.2. Air, oxygen, nitrogenand hydrogen are particularly preferred reactants.

The above deposition techniques to prepare composite coatings may beused with or without the conditions of lower substrate temperatures,high working gas pressures and low angles of incidence previouslydiscussed. One or more of these conditions is preferred to retain andenhance the amount of atomic disorder created in the coating.

It may be advantageous, prior to depositing an anti-microbial inaccordance with the present invention, to provide an adhesion layer onthe device to be coated, as is known in the art. For instance, for alatex device, a layer of Ti, Ta or Nb may be first deposited to enhanceadhesion of the subsequently deposited anti-microbial coating.

Anti-Microbial Powders

Anti-microbial powders, including nanocrystalline powders and powdersmade from rapidly solidified flakes or foils, can be formed with atomicdisorder so as to enhance solubility. The powders either as pure metals,metal alloys or compounds such as metal oxides or metal salts, can bemechanically worked or compressed to impart atomic disorder. Thismechanically imparted disorder is conducted under conditions of lowtemperature (i.e. temperatures less than the temperature ofrecrystallization of the material) to ensure that annealing orrecrystallization does not take place. The temperature varies betweenmetals and increases with alloy or impurity content.

Anti-microbial powders produced in accordance with this invention may beused in a variety of forms, for instance in topical creams, paints oradherent coatings. Alternatively, the powder may be incorporated into apolymeric, ceramic or metallic matrix to be used as a material formedical devices or coatings therefor.

Activation of Anti Microbial Materials

Irradiation of anti-microbial materials (powders, nanocrystallinepowders, foils, coatings or composite coatings of anti-microbial metals)which contain atomic disorder formed by any of the above-describedprocedures, will further activate or enhance the anti-microbial effect.Thus, even materials having a low level of atomic disorder may beactivated to an anti-microbial level.

Irradiation is performed with any low linear energy transfer form ofradiation, including beta, gamma and x-rays. Gamma radiation at a doseof 1 Mrad or greater is preferred. Since gamma radiation is anacceptable method of sterilization of medical devices, activation andsterilization may be achieved simultaneously through the irradiationprocess of the present invention.

The irradiation step is preferably conducted such that theanti-microbial material being irradiated is oriented generallyperpendicular to the incoming radiation (rather than parallel). Afurther enhancement of the anti-microbial effect can be achieved byconducting the irradiation step with a dielectric material adjacent to,or preferably sandwiched around the anti-microbial material. Exemplarydielectrics include oxides of Si, Ti, Ta and Al. Silicon oxide surfacesare preferred. It is believed that the dielectric material providesforward scattering of electrons into the anti-microbial coating.

Without being bound by the same it is believed that the irradiation stepis causing one or more of the following changes in the anti-microbialmaterial:

1) creating further atomic disorder, such as point defects;

2) enhancing oxygen adsorption/chemisorption to the surface of theanti-microbial material;

3) activating trapped dopant atoms or molecules such as oxygen to O⁺ orO₂ ⁻ ; and

4) creating broken or dangling bonds at the surface.

With respect to the second and third proposed mechanisms, it is possiblethat oxygen adsorption/chemisorption and/or activation creates a supersaturated concentration of O₂, O⁺ or O₂ ⁻ species in or on theanti-microbial metal surface, which results in the more rapiddissolution of the anti-microbial metal or species thereof into anaqueous environment through the generation of various chemical speciesof the anti-microbial metal, including oxides and hydroxides.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

A medical suture material size 2/0, polyester braid was coated bymagnetron sputtering an Ag-Cu-alloy onto the surface to a thickness of0.45 microns, using either argon gas working pressures of 7 mTorr or 30mT at 0.5 KW powder and a T/Tm ratio of less than 0.5.

The anti-microbial effect of the coatings was tested by a zone ofinhibition test. Basal medium Eagle (BME) with Earle's salts andL-glutamine was modified with calf/serum (10%) and 1.5% agar prior tobeing dispensed (15 ml) into Petri dishes. The agar containing Petriplates were allowed to surface dry prior to being inoculated with a lawnof Staphylococcus aureus ATCC#25923. The inoculant was prepared fromBactrol Discs (Difco, M.) which were reconstituted as per themanufacturer's directions. Immediately after inoculation, the materialsor coatings to be tested were placed on the surface of the agar. Thedishes were incubated for 24 h at 37° C. After this incubation period,the zone of inhibition was measured and a corrected zone of inhibitionwas calculated (corrected zone of inhibition=zone of inhibition-diameterof the test material in contact with the agar).

The results showed no zone of inhibition on the uncoated suture, a zoneof less than 0.5 mm around the suture coated at 7 mTorr and a zone of 13mm around the suture coated at 30 mTorr. Clearly the suture coated inaccordance with the present invention exhibits a much more pronouncedand effective anti-microbial effect.

EXAMPLE 2

This example is included to illustrate the surface structures which areobtained when silver metal is deposited on silicon wafers using amagnetron sputtering facility and different working gas pressures andangles of incidence (i.e. the angle between the path of the sputteredatoms and the substrate). All other conditions were as follows:deposition rate was 200 A°/min; ratio of temperature of substrate(wafer) to melting point of silver (1234° K.). T/Tm was less than 0.3.Argon gas pressures of 7 mTorr (a normal working pressure for metalcoatings) and 30 mTorr were used. Angles of incidence at each of thesepressures were 90° (normal incidence), 50° and 10°. The coatings had athickness of about 0.5 microns.

The resulting surfaces were viewed by scanning electron microscope. Asargon gas pressure increased from 7 to 30 mTorr the grain size decreasedand void volume increased significantly. When the angle of incidence wasdecreased, the grain size decreased and the grain boundaries became moredistinct. At 7 mTorr argon pressure and and angle of incidence of 10°,there were indications of some voids between the grains. The angle ofincidence had a greater effect on the surface topography when the gaspressure was increased to 30 mTorr. At 90°, the grain size varied from60-150 nm and many of the grains were separated by intergrain voidspaces which were 15-30 nm wide. When the angle of incidence wasdecreased to 50°, the grain size decreased to 30-90 nm and the voidvolume increased substantially. At 10°, the grain size was reduced toabout 10-60 nm and void volumes were increased again.

The observed nanometer scale changes in surface morphology andtopography are indications of atomic disorder in the silver metal. Whilenot being bound by the same, it is believed that such atomic disorderresults in an increase in the chemical activity due to increasedinternal stresses and surface roughness created by mismatched atoms. Itis believed that the increased chemical activity is responsible for theincreased level of solubility of the coatings when in contact with anelectrolyte such as body fluid.

The anti-microbial effect of the coatings was evaluated using the zoneof inhibition test as set out in Example 1. Each coated silicon waferwas placed on an individual plate. The results were compared to thezones of inhibition achieved when solid silver (i.e. greater than 99%silver) sheets, wires or membranes were tested. The results aresummarized in Table 1. It is evident that the pure silver devices andthe silver sputtered coating at 7 mTorr do not produce any biologicaleffect. However, the coatings deposited at a higher than normal workinggas pressure, 30 mTorr, demonstrated an anti-microbial effect, asdenoted by the substantial zones of inhibition around the discs.Decreasing the angle of incidence had the greatest effect onanti-microbial activity when combined with the higher gas pressures.

                  TABLE I                                                         ______________________________________                                        Anti-microbial effects of various silver and silver coated                    samples as determined using Staphylococcus aureus                                                          Working                                                                              Corrected                                                              Gas    Zone of                                            Percent  Angle of   Pressure                                                                             Inhibition                                Sample   Silver   Deposition (mTorr)                                                                              (mm)                                      ______________________________________                                        Silver Sheet-                                                                          99+      --         --     <0.5                                      rolled                                                                        Silver wire                                                                            99+      --         --     <0.5                                      (.0045")                                                                      Silver   99+      --         --     <0.5                                      membrane-                                                                     cast                                                                          Sputtered                                                                              99+      normal (90°)                                                                       7     <0.5                                      thin film                                                                     Sputtered                                                                              99+      50°  7     <0.5                                      thin film                                                                     Sputtered                                                                              99+      10°  7     <0.5                                      thin film                                                                     Sputtered                                                                              99+      normal (90°)                                                                      30     6.3                                       thin film                                                                     Sputtered                                                                              99+      50° 30     10                                        thin film                                                                     Sputtered                                                                              99+      10.sup.    30     10                                        thin film                                                                     ______________________________________                                    

EXAMPLE 3

Silicon wafers were coated by magnetron sputtering with an alloy of Agand Cu (80:20) at normal incidence at working gas pressures of 7 mTorrand 30 mTorr, all other conditions being identical to those set out inExample 2. As in Example 2, when the coatings were viewed by SEM, thecoatings formed at high working gas pressure had smaller grain sizes andlarger void volumes than did the coatings formed at the lower workinggas pressures.

Coatings which were similarly formed from a 50:50 Ag/Cu alloy weretested for anti-microbial activity with the zone of inhibition test setout in Example 1. The results are summarized in Table 2. Coatingsdeposited at low working gas pressure (7 mTorr) showed minimal zones ofinhibition, while the coatings deposited at high working gas pressure(30 mTorr) produced larger zones of inhibition, indicative ofanti-microbial activity.

                  TABLE 2                                                         ______________________________________                                        The anti-microbial effect of various sputter deposited silver-                copper alloys as determined using Staphylococcus aureus                                                            Corrected                                                 Angle of   Working Gas                                                                            Zone of                                         Percent   Deposition Pressure Inhibition                               Sample Silver    (°) (mTorr)  (mm)                                     ______________________________________                                        1      50        normal (90°)                                                                      7.5      <0.5                                     2      50        normal (90°)                                                                      30       16                                       3      50        10         30       19                                       ______________________________________                                    

EXAMPLE 4

A coating in accordance with the present invention was tested todetermine the concentration of silver ions released into solution overtime. One cm² silicon wafer discs were coated with silver as set forthin Example 2 at 7 mTorr and 30 mTorr and normal incidence to a thicknessof 5000 A°. Using the method of Nickel et al., Eur. J. Clin. Microbial,4(2), 213-218, 1985, a sterile synthetic urine was prepared anddispensed into test tubes (3.5 ml). The coated discs were placed intoeach test tubes and incubated for various times at 37° C. After variousperiods of time, the discs were removed and the Ag content of thefiltered synthetic urine was determined using neutron activationanalysis.

The results are set forth in Table 3. The table shows the comparativeamounts of Ag released over time from coatings deposited on discs at 7mTorr or 30 mTorr. The coatings deposited at high pressure were moresoluble than those deposited at low pressure. It should be noted thatthis test is a static test. Thus, silver levels build up over time,which would not be the case in body fluid where there is constant turnover.

                  TABLE 3                                                         ______________________________________                                        Concentration of silver in synthetic urine as a function of                   exposure time                                                                            Silver Concentration μg/ml                                                   Working Argon                                                                             Working argon                                        Exposure Time                                                                              gas pressure                                                                              gas pressure                                         (Days)       7 mTorr     30 mTorr                                             ______________________________________                                        0            ND1         ND                                                   1            0.89        1.94                                                 3            1.89        2.36                                                 10           8.14        23.06                                                ______________________________________                                         Note: Films were deposited at normal incidence (90°)                   1  ND (non detectable) <0.46 μg/ml                                    

EXAMPLE 5

This examples is included to illustrate coatings in accordance with thepresent invention formed from another noble metal, Pd. The coatings wereformed on silicon wafers as set forth in Example 2, to a thickness of5000 A°, using 7 mTorr or 30 mTorr working gas pressures and angles ofincidence of 90° and 10°. The coated discs were evaluated foranti-microbial activity by the zone of inhibition test substantially asset forth in Example 1. The coated discs were placed coating side upsuch that the agar formed a 1 mm surface coating over the discs. Themedium was allowed to solidify and surface dry, after which thebacterial lawn was spread over the surface. The dishes were incubated at37° C. for 24 h. The amount of growth was then visually analyzed.

The results are set forth in Table 4. At high working gas pressures, thebiological activity of the coating was much greater than that ofcoatings deposited at low pressure. Changing the angle of incidence(decreasing) improved the anti-microbial effect of the coating to agreater extent when the gas pressure was low than when it was high.

                  TABLE 4                                                         ______________________________________                                        Surface Control of Staphylococcus aureus by Sputter Deposited                 Palladium metal                                                                     Sputtering               Anti-microbial                                 Sample                                                                              Pressure  Angle of Deposition                                                                          Control                                        ______________________________________                                        1     7 mT      90° (normal incidence)                                                                More than 90% of                                                              surface covered by                                                            bacterial growth                               2     7 mT      10° (grazing inci-                                                                    20-40% of surface                                              dence)         covered by bacterial                                                          growth                                         3     30 mT     90° (normal incidence)                                                                Less than 10% sur-                                                            face covered by                                                               bacterial growth                               ______________________________________                                    

EXAMPLE 6

This example is included to illustrate the effect of silver depositiontemperature on the anti-microbial activity of the coating. Silver metalwas deposited on 2.5 cm sections of a latex Foley catheter using amagnetron sputtering facility. Operating conditions were as follows; thedeposition rate was 200 A° per minute; the argon working gas pressurewas 30 m Torr; and the ratio of temperature of substrate to meltingpoint of the coating metal silver, T/Tm was 0.30 or 0.38. In thisexample the angles of incidence were variable since the substrate wasround and rough. That is the angles of incidence varied around thecircumference and, on a finer scale, across the sides and tops of thenumerous surface features. The anti-microbial effect was tested by azone of inhibition test as outlined in Example 1.

The results showed corrected zones of inhibition of 0.5 and 16 mm aroundthe tubing coated at T/Tm values of 0.38 and 0.30 respectively. Thesections of Foley catheter coated at the lower T/Tm value were moreefficacious than those coated at higher T/Tm value.

EXAMPLE 7

This example is included to demonstrate an anti-microbial coating formedby DC magnetron sputtering on a commercial catheter. A teflon coatedlatex Foley catheter was coated by DC magnetron sputtering 99.99% puresilver on the surface using the conditions listed in Table 5. Theworking gases used were commercial Ar and 99/1 wt % Ar/O₂.

The anti-microbial effect of the coating was tested by a zone ofinhibition test. Mueller Hinton agar was dispensed into Petri dishes.The agar plates were allowed to surface dry prior to being inoculatedwith a lawn of Staphylococcus aureus ATCC#25923. The inoculant wasprepared from Bactrol Discs (Difco, M.) which were reconstituted as perthe manufacturer's directions. Immediately after inoculation, the coatedmaterials to be tested were placed on the surface of the agar. Thedishes were incubated for 24 hr. at 37° C. After this incubation period,the zone of inhibition was measured and a corrected zone of inhibitionwas calculated (corrected zone of inhibition=zone of inhibition-diameterof the test material in contact with the agar).

The results showed no zone of inhibition for the uncoated samples and acorrected zone of less than 1 mm for catheters sputtered in commercialargon at a working gas pressure of 5 mT. A corrected zone of inhibitionof 11 mm was reported for the catheters sputtered in the 99/1 wt % Ar/O₂using a working gas pressure of 40 mT. XRD analysis showed that thecoating sputtered in 1% oxygen was a crystalline Ag film. This structureclearly caused an improved anti-microbial effect for the coatedcatheters.

                  TABLE 5                                                         ______________________________________                                        Conditions of DC Magnetron Sputtering Used for                                Anti-Microbial Coatings                                                       Samples Sputtered in Samples Sputtered in                                     Commercial Argon     99/1 wt % Ar/O.sub.2                                     ______________________________________                                        Power      0.1 kW    Power        0.5 kW                                      Argon Pressure:                                                                          5 m Torr  Ar/O.sub.2 Pressure:                                                                       40 m Torr                                   Initial Substrate                                                                        20° C.                                                                           Initial Substrate                                                                          20° C.                               Temperature:         Temperature:                                             Cathode/Anode                                                                            40 mm     Cathode/Anode                                                                              100 mm                                      Distance:            Distance:                                                Film Thickness:                                                                          2500 Å                                                                              Film Thickness:                                                                            3000 Å                                  ______________________________________                                    

EXAMPLE 8

This example demonstrates silver coatings formed by are evaporation, gasscattering evaporation (pressure plating) and reactive are evaporation.Evaporation of 99.99% silver was performed onto silicon or aluminawafers at an initial substrate temperature of about 21° C., using theparameters as follows:

Bias: -100 V

Current: 20 Amp-hrs

Angle of incidence: 90°

Working Gas Pressure: 0.01 mT (arc), 26 mT Ar/H₂ 96:4 (gas scatteringevaporation), and

26 mT O₂ (reactive arc evaporation)

No corrected ZOI was observed for wafers coated at vacuum (arc).Pressure plating with a working gas atmosphere containing Ar and 4%hydrogen produced a 6 mm ZOI, while a working gas atmosphere of pureoxygen (reactive arc) produced an 8 mm ZOI. Film thicknesses of about4000 Angstroms were produced. The results indicate that the presence ofgases such as hydrogen and/or oxygen in the arc evaporation atmospherecause the coatings to have improved anti-microbial efficacy.

EXAMPLE 9

This example is included to illustrate composite materials to produceanti-microbial effects. A set of coatings were produced by RF magnetronsputtering zinc oxide onto silicon wafers as outlined below. The zincoxide coatings showed no zone of inhibition.

Coatings of Ag and ZnO were deposited to a total thickness of 3300Angstroms by sequentially sputtering layers of Ag with layers of ZnO,according to the conditions below, in a 75/25 wt % ratio. The coatingswere demonstrated to have no zone of inhibition when the zinc oxidelayers were about 100 Angstroms thick. However, films consisting ofislands of very thin to discontinuous layers of ZnO (less than 50Angstroms) in an Ag matrix (i.e. a composite film) had a 8 mm correctedzone of inhibition.

The conditions used to deposit ZnO were as follows: Working gas=argon;Working gas pressure=30 mT; Cathode-Anode distance: 40 mm; InitialSubstrate Temperature: 21° C.; Power: RF magnetron, 0.5 kW.

The conditions used to deposit the Ag were as follows: Workinggas=argon; Working gas pressure=30 mT; Cathode-Anode distance=40 mm;Initial Substrate Temperature=21° C.; Power=DC magnetron, 0.1 kW.

EXAMPLE 10

This example demonstrates the effects of cold working and annealingsilver and gold powders on the anti-microbial efficacy demonstrated by astandard zone of inhibition test. Cold working of such powders resultsin a defective surface structure containing atomic disorder whichfavours the release of ions causing anti-microbial activity. Theanti-microbial effect of this defective structure can be removed byannealing.

Nanocrystalline silver powder (crystal size about 30 nm) was sprinkledonto adhesive tape and tested. A zone of inhibition of 5 mm wasobtained, using the method set forth in Example 7. A 0.3 g pellet of thenanocrystalline Ag powder was pressed at 40,000 psi. The pellet produceda 9 mm zone of inhibition when tested for anti-microbial activity.Nanocrystalline silver powder was mechanically worked in a ball mill for30 sec. The resulting powder was tested for anti-microbial activity,both by sprinkling the worked powder on adhesive tape and applying tothe plates, and by pressing the powder into a pellet at the aboveconditions and placing the pellet on the plates. The zones of inhibitionobserved were 7 and 11 mm respectively. A pellet that had been pressedfrom the worked powder was annealed at 500° C. for 1 hour under vacuumconditions. A reduced zone of inhibition of 3 mm was observed for theannealed pellet.

These results demonstrate that nanocrystalline silver powder, whilehaving a small anti-microbial effect on its own, has an improvedanti-microbial effect by introducing atomic disorder by mechanicalworking of the powder in a ball mill or by pressing it into a pellet.The anti-microbial effect was significantly decreased by annealing at500° C. Thus, conditions of mechanical working should not include or befollowed by conditions such as high temperature, which allow diffusion.Cold mechanical working conditions are preferred to limit diffusion, forexample by working at room temperature or by grinding or milling inliquid nitrogen.

Silver powder, 1 micron particle size, was tested in a manner similar toabove. The Ag powder sprinkled onto adhesive tape and tested for a zoneof inhibition. No zone of inhibition was observed. The powder was workedin a ball mill for 30 seconds and sprinkled onto adhesive tape. A 6 mmzone of inhibition was observed around the powder on the tape. When theAg powder (as is or after mechanical working in the ball mill) waspressed into a 0.3 g pellet using 40,000 psi, zones of inhibition of 5and 6 mm respectively were observed. A pellet which was formed from theball milled powder and which was annealed at 500° C. for 1 hour hadsignificantly reduced anti-microbial activity. Initially the pellet hadsome activity (4.5 mm zone of inhibition) but after the pellet wastested a second time, no zone of inhibition was observed. A controlpellet which had not been annealed continued to give a zone ofinhibition greater than 4 mm even after 14 repeats of the test. Thisdemonstrates that an annealing step, following by mechanical working,limits the sustainable release of the anti-microbial silver species fromthe powders.

Nanocrystalline gold (20 nm crystals), supplied as a powder, was testedfor anti-microbial effect by sprinkling the powder onto adhesive tapeand using the zone of inhibition test. No zone of inhibition wasrecorded for the nanocrystalline gold powder. The gold powder waspressed into a 0.2 g pellet using 40,000 psi. A 10 mm zone of inhibitionwas observed. When the pressed pellets were subsequently vacuum annealedat 500° C. for 1 hour and the zone of inhibiton was found to be 0 mm.

The results showed that solubility and thus the anti-microbial efficacyof gold powders can be improved by a mechanical working process such aspressing a nanocrystalline material into a pellet. The anti-microbialactivity can be removed by annealing. Cold working is preferred.

Other gold powders including a 2-5 micron and a 250 micron particle sizepowder did not demonstrate an anti-microbial effect under the abovemechanical working conditions. It is believed that the small grain sizeof the nanocrystalline gold powder was an important cofactor which, withthe mechanical working, produced the desired anti-microbial effect.

EXAMPLE 11

This example is included to demonstrate a composite anti-microbialcoating formed by reactive sputtering (another example of compositefilms). Example 7 demonstrates that an anti-microbial coating of silvercan be obtained by sputtering in argon and 1% oxygen (0.5 kW, 40 mTorr,100 mm anode/cathode distance, and 20° C.-produced a zone of inhibitionof 11 mm).

When a working gas of argon and 20 wt % oxygen was used to sputteranti-microbial coatings under the conditions listed below, the zones ofinhibition ranged from 6 to 12 mm. This indicates that the provision ofa reactive atmosphere during vapour deposition has the result ofproducing an anti-microbial film over a wide range of deposition processparameters.

    ______________________________________                                        Sputtering Conditions                                                         ______________________________________                                        Target             99.99% Ag                                                  Working Gas:       80/20 wt % Ar/O.sub.2                                      Working Gas Pressure:                                                                            2.5 to 50 mTorr                                            Power:             0.1 to 2.5 kW                                              Substrate Temperature:                                                                           -5 to 20° C.                                        Anode/Cathode Distance                                                                           40 to 100 mm                                               Base Pressure:     less than 4 × 10.sup.-6 Torr                         ______________________________________                                    

EXAMPLE 12

This example demonstrates that the coatings of this invention have ananti-microbial effect against a broad spectrum of bacteria.

A total of 171 different bacterial samples encompassing 18 genera and 55species were provide by the Provincial Laboratory of Public Health forNorthern Alberta. These samples had been quick frozen in 20% skim milkand stored at -70° C. for periods ranging from several months to severalyears. Fastidious organisms which were unlikely to grow under conditionsused in standard Kirby-Bauer susceptibility testing were not used.

Each frozen sample was scraped with a sterile cotton swab to inoculate ablood agar plate (BAP). The plates were incubated overnight at 35° C.The following morning isolated colonies were subcultured onto fresh BAPsand incubated at 35° C. overnight. The next day, the organisms weresubjected to Kirby-Bauer susceptibility testing as described below.

Four to five colonies (more if colonies were small) of the samemorphological type were selected from each BAP subculture and inoculatedinto individual tubes containing approximately 5 mL of tryptic soy broth(TSB). The broths were incubated at 35° C. for approximately 2 to 3hours. At this time, the turbidity of most of the broth cultures eitherequalled or exceeded that of a 0.5 McFarland standard. The more turbidsamples were diluted with sterile saline to obtain a turbidity visuallycomparable to that of the standard. To aid in the visual assessment ofturbidity, tubes were read against a white background with contrastingblack line.

A small number of the organisms (Streptococcus and Corynebacterium) didnot grow well in TSB. The turbidity of these broths, after incubation,was less than that of the 0.5 McFarland standard. Additional coloniesfrom the BAP subcultures were inoculated to these tubes to increase theturbidity to approximate that of the standard.

Within 15 minutes of adjusting the turbidity of the bacterialsuspensions a sterile cotton swab was dipped into each broth. Excessfluid was removed by rotating the swab against the rim of the tube. Theinoculum was applied to a Mueller Hinton (MH) agar plate by streakingthe swab evenly in three directions over the entire agar surface. Three1 cm×1 cm silver coated silica wafer squares were applied to each MHplate and the plates were inverted and incubated overnight at 35° C. Thecoatings had been sputtered under the following conditions, whichthrough XFD analysis were shown to be silver/silver oxide compositefilms:

    ______________________________________                                        Target:               99.99% Ag                                               Working gas:          Ar/O.sub.2 80/20                                        Working gas pressure: 40 mT                                                   Power:                0.1 kW                                                  Temperature of Deposition                                                                           20° C.                                           Base pressure         2 × 10.sup.-6 Torr                                Cathode/anode distance                                                                              40 mm                                                   ______________________________________                                    

BAP cultures of control organisms were provided by the ProvincialLaboratory and included: Staphylococcus aureus ATCC 25923; Pseudomonasaeruginosa ATCC 27853; Escherichia coli: ATCC 25922; and Enterococcusfaecalis ATCC 29212 to check the quality of the MH agar. These cultureswere treated in a like manner to the test organisms except that standardantibiotic discs rather than silver coated wafers were applied to thebacterial lawns on the MH agar. These organisms demonstrated that theMII agar was suitable for standard ZOI tests.

After 16 to 18 hours of incubation at 35° C. zones of inhibition aroundthe silver wafers or antibiotic discs were measured to the nearest mm.Corrected zones were calculated by subtracting the size of the wafer (1cm) from the size of the total zone. Representative zone of inhibitionresults are shown in Table 7.

                  TABLE 7                                                         ______________________________________                                        The Sensitivity of a Broad Range of Microorganisms to Silver*                 Coated Silicon Wafers                                                                                     Corrected Zone                                    Organism           Source   of Inhibition (mm)                                ______________________________________                                        Staphylococcus epidermidis RC-455                                                                blood    10                                                Bacillus licheniformis R-2138                                                                    tibia    6                                                 Corynebacterium sp R-594                                                                         leg      10                                                Listeria monocytogenes R-590                                                                     blood    5                                                 Enterococcus faecalis SR-113                                                                     bone     5                                                 Straptococcus bovis SR-62                                                                        blood    10                                                Escherichia coli R-1878                                                                          urine    11                                                Klebsiella ozonae R-308/90                                                                       abdomen  10                                                Enterobacter cloacae R-1682                                                                      unknown  8                                                 Proteus vulgaris 3781                                                                            urine    4                                                 Providencia stuartii U-3179                                                                      urine    8                                                 Citrobacter freundii U-3122/90                                                                   urine    7                                                 Salmonella typhimirium ER-115-4                                                                  urine    6                                                 Serparia marcescens R-850                                                                        sputum   6                                                 Pseudomonas aeruginosa U-3027                                                                    urine    10                                                Xanthomonas maltophila 90-10B                                                                    unknown  9                                                 Aeromonas caviae R-1211                                                                          wound    5                                                 Branhamella catarrhalis R-2681                                                                   unknown  12                                                ______________________________________                                         Silver deposition*                                                       

EXAMPLE 13

This example demonstrates the use of tantalum as an adhesive layer forcoatings of this invention. Tantalum is well known as a material which,in the form of an interlayer, improves adhesion of thin films tosubstrates. In this example test sections including a group of stainlesssteel (316) (1×1 cm) and silicon (1.7×0.9 cm) coupons and sections oflatex tubing (5 cm) were cleaned in ethanol and then half of the testsections were coated (by sputtering) with a thin layer (approx. 100Angstroms) of Ta before an anti-microbial silver film was deposited onthem. The second group of the test sections were only coated with theanti-microbial Ag film. Coating conditions are listed below. While alltest sections had similar anti-microbial activity, the Ta coated testsections had much better adhesion properties than did the untreated testsections. Adhesion properties were determined using ASTM methodD3359-87, a standard test method for measuring adhesion.

    ______________________________________                                        Sputtering Conditions                                                         ______________________________________                                        Target:              99.99% Ta                                                Working Gas:         99/1 wt % Ar/O.sub.2                                     Working Gas Pressure:                                                                              10 mTorr                                                 Power:               0.5 kW                                                   Cathode/Anode Distance:                                                                            100 mm                                                   Substrate Temperature:                                                                             20° C.                                            Target:              99.99% Ag                                                Working Gas:         99/1 wt % Ar/O.sub.2                                     Working Gas Pressure:                                                                              40 mTorr                                                 Power:               0.5 kW                                                   Cathode/Anode Distance:                                                                            100 mm                                                   Substrate Temperature:                                                                             20° C.                                            ______________________________________                                    

EXAMPLE 14

DC magnetron sputtering was used to deposit silver from a 99.98% purecathode onto silicon and alumina wafers with commercial argonmoisturized with water as the working gas. The argon was moisturized bypassing it through two flasks containing 3 liters of room temperaturewater and one empty flask set up with glass wool to absorb any freeliquid before the gas entered the sputtering unit.

The conditions of sputtering and the results of the standard zone ofinhibition test performed on the sputtered silver films are shown below.Silver films which normally had no anti-microbial properties whendeposited using argon that had not been treated with water yielded acorrected zone of inhibition of up to 8 mm when sputtered using aargon/water vapour mixture as the working gas.

                  TABLE 8                                                         ______________________________________                                        Conditions used for DC Magnetron Sputtering of                                Anti-Microbial Caotings                                                                Working          Substrate                                                                            Anode/                                       Working  Gas              Tempera-                                                                             Cathode                                                                              Correct-                              Gas      Pressure Power   ture   Distance                                                                             ed Z01                                ______________________________________                                        Commercial                                                                             10 mTorr 0.5 kW  -10° C.                                                                       100 mm 0 mm                                  Argon                                                                         Ar passed                                                                              10 mTorr 0.5 kW  -10° C.                                                                       100 mm 8 mm                                  through                                                                       H.sub.2 O                                                                     ______________________________________                                    

EXAMPLE 15

This example is included to illustrate the method of activating coatingswith radiation, in accordance with another aspect of the presentinvention.

A series of 1.9×0.7 cm silicon wafers were coated with 3000 Å coatingsof silver metal using DC magnetron sputtering under the followingconditions:

    ______________________________________                                        Sputtering Conditions                                                         ______________________________________                                        Target               99.99% Ag                                                Working Gas          99/1 wt % Ar/O.sub.2                                     Working Gas pressure 40 mTorr                                                 Power                0.5 kW                                                   Substrate Temperature                                                                              21° C.                                            Anode/Cathode Distance                                                                             100 mm                                                   ______________________________________                                    

The coated wafers were divided into 4 groups and irradiated with varyingdoses of gamma radiation--0, 1, 2 and 4 megarad doses--from a ⁶⁰ Cosource at Isomedix Inc., Morton Grove, Ill., U.S.A. The samples wereplaced generally perpendicular to the incoming radiation. Afterirradiation, the samples were tested for biological activity(anti-microbial effect) using a standard zone of inhibition test onMueller Hinton Agar (Difco, Mi) with S. aureus (ATCC #25923), as set outin previous examples. The results are summarized in Table 9.

                  TABLE 9                                                         ______________________________________                                        Effects of Gamma Radiation on Biological Activiy of                           Anti-Microbial Coatings                                                       Gamma Radiation Dose                                                          (megarads)     Corrected Zone of Inhibition (mm)                              ______________________________________                                        0              11                                                             1              14                                                             2              17                                                             4              20                                                             ______________________________________                                    

The results generally show a log dose response relationship between theradiation dose and the observed biological response to the wafers. Thisillustrates that the gamma radiation has further activated the coatingsof the present invention to enhance the anti-microbial effect.

The experiment was repeated with the anti-microbial films being orientedgenerally parallel to the incoming radiation. This orientationsubstantially reduced the level of activation of the anti-microbialcoatings, such that no increase in the zone of inhibition was observedrelative to controls which had not been irradiated.

EXAMPLE 16

This example is included to illustrate activation of the anti-microbialcoatings in accordance with the present invention with gamma radiationusing a dielectric material adjacent to the material during irradiation.

A number of 1"×1" pieces of high density polyethylene mesh (such as usedin burn wound dressings) were sputter coated with silver metal under thesame conditions as set forth in Example 15 with the exception that thepower was 0.1 kW. The coated mesh was then irradiated (perpendicularorientation) as set forth in Example 15 at 4 megarads. The biologicalactivity was then tested, as set out in Example 15. Control mesh samples(silver coated, no irradiation) gave a 10 mm ZOI(corrected), while theirradiated samples gave a 14 mm ZOI(corrected).

Further samples of the coated mesh were irradiated while sandwichedbetween two 1"×1" silicon wafers having a 1000 Å thermally grown oxidelayer, as supplied by the Alberta Microelectronics Centre, Edmonton,Alberta. This mesh sample was tested for biological activity and wasfound to produce a 26 mm ZOI(corrected). Without being bound by thesame, it is believed that the silicon wafers provide a source ofelectrons which are forward scattered to the anti-microbial coatings,further enhancing the anti-microbial effect.

Bulk silver sheet metal was tested to determine whether it could beactivated to produce an anti-microbial effect by gamma irradiation. Thebulk silver sheet metal samples were annealed at 140° C. for 90 minutesin air and then irradiated with a 4 megarad dose. The samples weretested for biological activity, but no ZOI was produced. This resultappears to indicate that bulk silver, in its normal ordered crystallinestate, has too few atomic defects to be activated in accordance with theprocess of the present invention.

EXAMPLE 17

This example is included to illustrate that anti-microbial coatingscontaining atomic disorder at a level that is insufficient to produce ananti-microbial effect can be further activated by gamma irradiation, inaccordance with the present invention.

Silver films were sputtered onto silicon wafers, as set forth in Example15, except that the gas pressure was reduced from 40 mTorr to 5 mTorr,resulting in less atomic disorder in the coatings. The silver films werethen irradiated with a 4 Mrad dose of gamma radiation, as in Example 15.The irradiated and control films (not irradiated) were tested forbiological activity. The control films produced only 1 mmZOI(corrected), while the irradiated coatings produced 10 mmZOI(corrected). This result demonstrates that anti-microbial materialsprepared under conditions such that they contain atomic disorder at alevel insufficient to produce an anti-microbial effect can be activatedso as to be anti-microbial by irradiating with a source of gammaradiation.

All publications mentioned in this specification are indicative of thelevel of skill of those skilled in the art to which this inventionpertains. All publications are herein incorporated by reference to thesame extent as if each individual publication was specifically andindividually indicated to be incorporated by reference.

The terms and expressions in this specification are used as terms ofdescription and not of limitation. There is no intention, in using suchterms and expressions, of excluding equivalents of the featuresillustrated and described, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

We claim:
 1. A method of forming an anti-microbial material containingone or more anti-microbial metals, said method comprising:creatingatomic disorder in a material containing one or more anti-microbialmetals under conditions which limit diffusion for retaining atomicdisorder therein to provide sustained release of atoms, ions, moleculesor clusters of at least one of the metals into an alcohol or water basedelectrolyte at an enhanced rate relative to the material in its normalordered crystalline state; and irradiating the material with a lowlinear energy transfer form of radiation to release at least oneanti-microbial metal at a concentration sufficient to provide alocalized anti-microbial effect.
 2. The method as set forth in claim 1,wherein the anti-microbial metal is selected from the group consistingof Ag, Au, Pt, Pd, Ir, Sn, Cu, Sb, Bi, Zn, alloys thereof, and compoundsthereof.
 3. The method as set forth in claim 2, wherein the material isa powder or foil of one or more of the anti-microbial metals, andwherein the atomic disorder is formed by cold working of the powder orfoil.
 4. The method as set forth in claim 2, wherein the material isformed as a coating on a substrate by vapour deposition under conditionswhich limit diffusion during deposition and which limit annealing orrecrystallization following deposition.
 5. The method as set forth inclaim 3, wherein the material is a nanocrystalline powder.
 6. The methodas set forth in claim 4, wherein the material is formed by vacuumevaporation, sputtering, magnetron sputtering or ion plating.
 7. Themethod as set forth in claim 4, wherein the coating is formed bymagnetron sputtering at conditions such that the ratio of thetemperature of the surface being coated to the melting point of theanti-microbial material being deposited is less than about 0.5, and theworking gas pressure is greater than about 10 mT.
 8. The method as setforth in claim 6, wherein the anti-microbial material is a compositecoating formed by co-, sequentially or reactively depositing ananti-microbial metal in a matrix with atoms or molecules of a differentmaterial to create atomic disorder in the matrix, said differentmaterial being deposited as one or more members selected from the groupconsisting of oxygen, nitrogen, hydrogen, boron, sulphur or halogenabsorbed or trapped in the matrix from the atmosphere of the vapourdeposition; an oxide, nitride, carbide, boride, halide, sulphide orhydride of an anti-microbial metal; and an oxide, nitride, carbide,boride, halide, sulphide or hydride of an inert biocompatible metalselected from the group consisting of Ta, Ti, Nb, V, Hf, Zn, Mo, Si, andAl.
 9. The method as set forth in claim 8, wherein the anti-microbialmetal is silver and said different material is one or both of silveroxide and atoms or molecules containing oxygen trapped or absorbed inthe matrix from the atmosphere of the vapour deposition.
 10. The methodas set forth in claim 8, wherein the coating is formed by magnetronsputtering at conditions such that the ratio of the temperature of thesurface being coated to the melting point of the anti-microbial materialbeing deposited is less than about 0.5, and the working gas pressure isgreater than about 10 mT.
 11. The method as set forth in claim 9,wherein the coating is formed by magnetron sputtering at conditions suchthat the ratio of the temperature of the surface being coated to themelting point of the anti-microbial material being deposited is lessthan about 0.5, and the working gas pressure is greater than about 10mT.
 12. The process as set forth in claim 1, 3 or 6, wherein the form ofradiation is selected from gamma, beta and x-rays.
 13. The process asset forth in claim 1, 3 or 6, wherein the source of radiation is gammaradiation, used at a dose of greater than about 1 Mrad.
 14. The processas set forth in claim 1, 3 or 6, wherein the anti-microbial materialbeing irradiated is oriented substantially perpendicular to the incomingradiation.
 15. The process as set forth in claim 1, 3 or 6, wherein thematerial is placed adjacent to a dielectric material during irradiation.16. The process as set forth in claim 1, 3 or 6, wherein the material issandwiched between silicon oxide surfaces during irradiation.